1. Field of the Invention
This invention relates to the drilling or boring of wells in the earth. More specifically, a method is provided for analyzing and controlling the effects of accumulation of cuttings in the wellbore when drilling at an angle greater than about 50 degrees to vertical.
2. Description of Related Art
The process of boring or drilling in the earth produces particles of rock which must be removed from the borehole. One of the functions of the drilling fluid used in the drilling process is to carry these "cuttings" from the bit where they are created to the surface of the earth. In vertical or near-vertical wells, the drilling fluid is normally formulated to have viscosity high enough to decrease the settling velocity of cuttings to a value much less than the upward velocity of drilling fluid in the hole, so the cuttings will be efficiently carried from the well. Gel strength of the fluid is also normally formulated to prevent rapid fall of cuttings in the wellbore when fluid circulation is interrupted.
In recent years, there has been a major upswing in the drilling of wells in directions other than vertical. "High-angle" wells are drilled for hydrocarbon production from platforms constructed offshore and from pads built in the arctic. "Horizontal wells," a sub-class of high-angle wells, are drilled at angles near 90 degrees to vertical for a variety of reasons related to hydrocarbon production; they may also be drilled for environmental remediation and other purposes. Some high-angle wells may terminate at a location displaced thousands of feet horizontally from the surface location of the well. There is a very large economic incentive at times to push this horizontal displacement to the maximum distance achievable so that additional hydrocarbons can be recovered from existing surface facilities.
It has been recognized for many years that removal of the cuttings from the wellbore during drilling of high-angle wells poses a special problem. When the cuttings settle by force of gravity to the bottom of the hole, a "bed" of solids is formed along the bottom of the hole. The bed can become especially significant in larger size holes, where fluid velocities are lower. Experience from drilling high-angle wells shows that pipe sticking and related drilling problems are especially frequent in the larger holes (171/2-inch and 121/4-inch holes) drilled at angles above about 40-50 degrees.
Build-up of a bed of cuttings in the high-angle portion of wellbores, or, stated another way, failure to achieve sufficient "hole cleaning," can cause several types of problems. One problem at times may be that the amount of torque required to rotate the drill string increases to limit the ability to drill the well to the target location. Another is that there is difficulty in withdrawing and placing the drill string in the well. In the most unfavorable scenario, the amount of pull required to withdraw the drill string from the well increases to the point that the pipe is stuck in the well. This condition can be very expensive to remedy. A single stuck pipe incident may cost over one million dollars. It is estimated that stuck pipe costs in industry are in the range from 100 to 500 million dollars per year. In attempts to avoid such problems from insufficient hole-cleaning, drilling operators often include such maneuvers as "washing and reaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is introduced into the wellbore, and "backreaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is withdrawn from the wellbore. Other operations such as "wiper trips" or "pumping out of the hole" are performed to attempt to control the amount of cuttings accumulated in the wellbore. All these operations require time and can very significantly add to the cost of drilling a high-angle well.
In addition to excessive forces on the drill string, solids accumulation in the wellbore can also cause interference with running casing in the hole after drilling is complete and can cause excessive circulation pressures leading to loss of drilling fluid from the well bore.
Several studies in university and industry laboratories in recent years have been directed to hole-cleaning in high-angle wells. Recent reports have been published, for example, by J. T. Ford et al, SPE 20421, Society of Petroleum Engineers (Richardson, Tex.), 1990, and by T. R. Sifferman and T. E. Becker, SPE 20422, Society of Petroleum Engineers, 1990. There is no general agreement among investigators, however, on the factors which will cause bed height of cuttings in a high-angle well to increase or decrease. No methods are known that calculate the correct height of the bed of cuttings in a high-angle well.
U.S. Pat. No. 4,791,998 is directed to a method of avoiding stuck drilling equipment. The method is based on a statistical analysis of drilling variables in wells drilled in the same area, comparing wells in which sticking was experienced and not experienced, and modifying variables in a drilling well toward those conditions which the mathematical analysis indicates will not cause sticking.
There is a long-felt need for a method to select the most cost-effective values of drilling variables and conditions to drill a high-angle well where cuttings may accumulate in the wellbore. This method should allow a prediction of the height of a bed of cuttings in a well as drilling variables are adjusted. The method should consider, along with bed height, the size and configuration of drilling equipment in the hole and their effect on the likelihood of sticking the drill string in the hole. Such method is needed for planning the material and equipment which will be provided to drill a well and to adjust drilling variables while the well is being drilled. The method should be susceptible to further refinement by hindcasting drilling data from wells previously drilled to determine the relationship between well conditions and frequency of drilling problems. With such relationship, the risk of encountering drilling problems can be assessed and, when necessary, reduced with increasing confidence in wells of interest.